1. Field of the Invention
The present invention relates generally to CDMA receivers and CDMA receiving methods. In particular, the present invention relates to a CDMA receiver in which blind rate transmission is used and the pseudonoise (PN) codes used for spreading at transmit sites and those used for despreading at receive sites are structured in hierarchical order.
2. Description of the Related Art
In a spread spectrum channel, receiving and demodulating a signal that has been subject to spread spectrum modulation requires that the same pseudonoise (PN) code sequence be generated in the receiver, and correlated with received signals to extract data symbol values. These PN code sequences are hierarchically structured so that a PN code sequence corresponding to a low symbol rate can be determined from a PN code sequence corresponding to a higher symbol rate. Spread spectrum signals are transmitted at a symbol rate which is selected from a plurality of predetermined symbol rates so that the symbol rate selected for a given frame may differ from the rate used in another frame. No information is transmitted to receivers regarding the transmitted symbol rate. Rather, it is up to the receivers to determine the transmitted symbol rate. This mode of transmission is called blind-rate transmission.
In the current blind-rate transmission where two symbol rates are used, the transmitted spread spectrum signal is correlated, at a receive site, with a PN code sequence that corresponds to the high symbol rate to produce a first despread signal and the despread signal is decoded and tested for error. If an error is detected, a second despread signal is produced corresponding to the lower symbol rate and decoded and tested again. If an error is detected again, an alarm is given. Since the transmitted symbol rate is unknown, the receiver would frequently attempt to repeat the decoding process whenever the transmitted symbol rate varies from one frame to another. Since the decoding process is usually provided by a forward error correction (FEC) decoder which is complex, the frequent attempts to decode despread signals place a significant burden on the FEC decoder, resulting in an increase both in decoding time and power consumption. This is particularly disadvantageous for mobile spread spectrum receivers.
It is therefore an object of the present invention to provide a spread spectrum receiver and method that reduces the number of decoding processes.
The stated object is attained by estimating the transmitted symbol rate from a received spread spectrum signal.
According to a first aspect, the present invention provides a spread spectrum receiver for receiving a spread spectrum signal transmitted at one of first and second symbol rates, comprising. despreading circuitry for receiving the spread spectrum signal and initially producing therefrom a first despread signal corresponding to the first symbol rate, the despreading circuitry being responsive to a command signal for subsequently producing a second despread signal corresponding to the second symbol rate, symbol rate estimation circuitry for estimating, from the first despread signal, the transmitted symbol rate of the received spread spectrum signal as one of the first and second symbol rates and causing the despreading circuitry to continue producing the first despread signal if the transmitted symbol rate is estimated as the first symbol rate and supplying the command signal to the despreading circuitry if the transmitted symbol rate is estimated as the second symbol rate, and decoding circuitry for decoding the first and second despread signals produced by the despreading circuitry.
According to a second aspect, the present invention provides a method of receiving a spread spectrum signal transmitted at one of first and second symbol rates, comprising the steps of (a) despreading the spread spectrum signal to initially produce a first despread signal corresponding to the first symbol rate, (b) estimating, from the first despread signal, the transmitted symbol rate of the received spread spectrum signal as one of the first and second symbol rates, (c) repeating the step (a) if the transmitted symbol rate is estimated as the first symbol rate, ( d) producing a second despread signal corresponding to the second symbol rate if the transmitted symbol rate is estimated as the second symbol rate, and (e) decoding the first and second despread signals.
According to a third aspect, the present invention provides a method of receiving a spread spectrum signal transmitted at one of first and second symbol rates, comprising the steps of (a) producing a first despread signal corresponding to the first symbol rate from the received spread spectrum signal, (b) estimating, from the first despread signal, the transmitted symbol rate of the received spread spectrum signal as one of the first and second symbol rates, (c) if the transmitted symbol rate is estimated as the first symbol rate, repeating the step (a), (d) decoding the first despread signal and performing a test on the decoded signal, (e) if the test indicates that the decoded signal is invalid, producing a second despread signal corresponding to the second symbol rate, (f) decoding the second despread signal and performing a test on the decoded signal, (g) if the transmitted symbol rate is estimated as the second symbol rate, producing the second despread signal, and repeating the step (f), and (h) if the test indicates that the decoded signal is invalid, producing the first despread signal from the received spread spectrum signal, and decoding the first despread signal.